Manage your subscription

Science: Pair of planets deepens pulsar puzzle

One of the brightest pulsars in the sky has two planets orbiting it,
claim two Russian astronomers. If they are right, this will be only the
second pulsar confirmed to have planets.

The pulsar, called PSR 0329+54, is in Camelopardalis, a faint constellation
close to Ursa Major and Cassiopeia. Its planets were discovered when T.
V. Shabanova of the Pushchino Radio Astronomy Observatory analysed 25 years
of pulsar timing data collected by Russian and American observers. She found
that there were subtle variations in the timing of radio bursts emitted
by the pulsar. This indicates the presence of two planets, one orbiting
the pulsar every 1110 days, the other every 6140.

Shabanova calculates that the inner planet is 2.3 astronomical units
from the pulsar and at least 0.3 times as massive as Earth, while the outer
planet is at least two Earth masses and orbits at 7.3 astronomical units
(an astronomical unit is the average distance between the Sun and Earth).

The Camelopardalis pulsar differs greatly from the other pulsar with
planets, PSR 1257+12 in the constellation of Virgo (see ‘Puzzle of the pulsar
planets,’ New Scientist, 18 July 1992). The Virgo pulsar, which emits radio
pulses every 6.2 milliseconds, has two planets – one circling every 67 days,
the other every 98. It formed about a billion years ago from a star that
consumed matter from a companion star and then became a supernova. The
planets probably formed from the debris of the companion star, left over
from the explosion.

Advertisement

The Camelopardalis pulsar has a much longer period. According to Steve
Thorsett of the California Institute of Technology in Pasadena, this is
what would be expected if the pulsar is the relic of a supernova explosion
that occurred only about five million years ago. This is too short a time
for planets to have formed from orbiting debris, he says. If the planets
are real, they must have existed before the star exploded and survived the
cataclysm.

So much mass is blown into space by a supernova explosion that most
companion objects should be blown away too. But, according to Thorsett,
about 1.5 per cent of such objects should stay in orbit around the pulsar.
Because about 600 pulsars are known, ‘the odds are right for finding a companion’,
says Thorsett.

R. D. Dagkesamansky of Pushchino and Y. P. Shitov of the Pulsar Laboratory
at the Lebedev Physics Institute in Moscow report that the larger planet
has an eccentricity (a measure of the elongation of an elliptical orbit)
of 0.23 (International Astronomical Union Circular 5930). This large value
is just what is expected for an object still in orbit after a supernova
explosion.

American astronomers are reacting cautiously to the Russian report.
The Russians are not the first to suggest that the Camelopardalis pulsar
has a planet. In 1979, other astronomers claimed that the timing of the
radio pulses varied over a period of 1105 days, but no one could confirm
this.

Thorsett notes that young pulsars are prone to spin instabilities which
make the radio pulses appear as if they are being influenced by orbiting
planets. As observations of the outer planet extend over only 1.5 orbital
periods, says Thorsett, we will have to wait 10 to 15 years – for completion
of the second period – before we have further evidence that the planet exists.